Recently, laser diodes utilizing compound semiconductor materials such as GaAs, etc. have been developed as optical devices for transmitting large-volume information at a high speed. Such optical devices comprise multi-layer thin films having hetero structures, and the thin films are generally formed by a gas-phase epitaxial method or a molecular beam epitaxial method. Because the optical devices are strongly required to have such characteristics as stability and long life, epitaxial layers should have as little defect density as possible. Accordingly, it is required that semiconductor substrates on which epitaxial layers are formed have a low dislocation density.
Substrates for devices are generally cut out from semiconductor single crystals. Methods for producing semiconductor single crystals include a gas phase growing method, a liquid phase growing method, and a solid phase growing method, and single crystals of compound semiconductors are mostly produced by the liquid phase growing method. Methods for growing single crystals from seed crystals by solidifying starting material melts, as one type of the liquid phase growth method, include a horizontal Bridgman method, a vertical Bridgman method, a gradient freezing method (GF method), and a Czochralski method (CZ method) and its improved method such as a liquid-encapsulated Czochralski method (LEC method), etc.
Recently, much attention is paid to a vertical Bridgman method (VB method) as a method for producing GaAs crystals having as large diameters as more than 3 inches (76.2 mm) and a low dislocation density, instead of the LEC method. For instance, when a single crystal of a GaAs compound semiconductor is formed by the VB method, starting materials consisting of Ga and As or GaAs are charged into a crystal-growing container having a bottom on which a GaAs seed crystal is disposed, the crystal-growing container containing a starting material melt obtained by heating is moved in a space having a temperature gradient in a vertical direction, so that crystallization occurs from the lower side (from the side of the seed crystal) toward the upper side. As a result, a single crystal grows from the seed crystal in a direction perpendicular to its surface. Thus, the VB method can form high-quality, large-diameter single crystals of compound semiconductors with few crystal defects.
The high-quality, large-diameter single crystals of compound semiconductors can also be produced by a vertical gradient freeze (VGF) method in place of the VB method. The VGF method is essentially the same as the VB method, except that with a container containing a single crystal at its lower end located at a fixed position, the furnace temperature is lowered while maintaining a vertical temperature gradient, so that a single crystal is caused to grow in a direction perpendicular to the surface of a seed crystal.
Low-dislocation substrates are desired for compound semiconductor lasers and LEDs from the viewpoint of device life, efficiency, etc. The reason therefor is that because the above device is operated at a high current density, dislocation existing in the substrate would lead to the generation of heat therein, resulting in the deterioration of devices. However, p-type GaAs substrates produced by the conventional VB method have an average dislocation density of about 1000 cm−2, and it is thus difficult to produce low-dislocation single crystals at high yield.
As a method for achieving low dislocation in the p-type GaAs substrates, JP 2000-86398 A discloses a method for achieving an average dislocation density of 500 cm−2 or less by doping the p-type GaAs substrates with Si and B in addition to Zn, a p-type dopant. However, because impurity-hardening effects are obtained only by Si and B in the above method, it is difficult to achieve an average dislocation density of 100 cm−2 or less.